CardDAV backend to NextCloud not working

I have a recently installed NextCloud instance with Calendar & Contacts app. I can connect to it with a CardDAV client (iPhone) and sync my contacts. However, I am trying to merge e-mail, contact and calendar accounts into one using EAS, hence Z-Push.
I am using the combined backend with IMAP and CardDAV.
E-mail sync works, contacts do not. The contact list is empty and when I add a new contact the following is logged.